Fuel cells are devices that convert fluid streams containing a fuel, for example hydrogen, and an oxidizing species, for example, oxygen or air, to electricity, heat and reaction products. Such devices comprise an anode, where the fuel is provided; a cathode, where the oxidizing species is provided; and an electrolyte separating the two. The anode-electrolyte-cathode body is called the catalyst coated membrane herein. The fuel and/or oxidant typically is a liquid or gaseous material. The electrolyte is an electronic insulator that separates the fuel and oxidant. It provides an ionic pathway for the ions to move between the anode, where the ions are produced by reaction of the fuel, to the cathode, where they are used to produce the product. The electrons produced during formation of the ions are used in an external circuit, thus producing electricity. As used herein, fuel cells may include a single cell comprising only one anode, one cathode and an electrolyte interposed therebetween, or multiple cells assembled in a stack. In the latter case there are multiple separate anode and cathode areas wherein each anode and cathode area is separated by an electrolyte. The individual anode and cathode areas in such a stack are each fed fuel and oxidant, respectively, and may be connected in any combination of series or parallel external connections to provide power.
Additional components in a single cell or in a fuel cell stack may optionally include means to distribute the reactants across the anode and cathode, including, but not limited to porous gas diffusion media. Various sealing materials used to prohibit mixing of the various species may also be used. As used herein, the membrane electrode assembly (MEA) comprises the catalyst coated membrane and such gas diffusion media and sealing materials. Additionally, so-called bipolar plates, which are plates with channels to distribute the reactant may also be present.
A Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a type of fuel cell where the electrolyte is a polymer electrolyte. Other types of fuel cells include Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), etc. As with any electrochemical device that operates using fluid reactants, unique challenges exist for achieving both high performance and long operating times. In order to achieve high to performance it is necessary to reduce the electrical and ionic resistance of components within the device. Recent advances in the polymer electrolyte membranes have enabled significant improvements in the power density of PEMFCs. Steady progress has been made in various other aspects including lowering Pt loading, extending membrane life, and achieving high performance at different operating conditions. However, many technical challenges are still ahead. One of them is for the membrane electrode assembly to meet the lifetime requirements for various potential applications. These range from hundreds of hours for portable applications to 5,000 hours or longer for automotive applications to 40,000 hours or longer in stationary applications.
Although all of the materials in the fuel cell may be subject to degradation during operation, the integrity and health of the membrane is particularly important. Should the membrane degrade during fuel cell operation, it tends to become thinner and weaker, thus making it more likely to develop holes or tears. Should this occur, the oxidizing gas and fuel may mix internally potentially leading to internal reactions. Because such an internal reactions may ultimately cause damage to the entire system, the fuel cell must be shut down. One well known approach to assessing the health of a membrane is to measure the amount of fluoride ions in the product water of the fuel cell. Higher values of this so-called fluoride release rate are indicative of more attack of the membrane, and therefore are associated with membranes that have lower durability. Correspondingly, lower fluoride release rates are indicative of a healthier membrane, one more likely to have longer life.
As is well known in the art, decreasing the thickness of the polymer electrolyte membrane can reduce the membrane ionic resistance, thus increasing fuel cell power density. However, reducing the membranes physical thickness can increase the susceptibility to damage from other device components leading to shorter cell lifetimes. Various improvements have been developed to mitigate this problem. For example, U.S. Pat. No. RE 37,307, U.S. Pat. No. RE 37,701,
US Application No. 2004/0045814 to Bahar et al., and U.S. Pat. No. 6,613,203 to Hobson, et. al. show that a polymer electrolyte membrane reinforced with a fully impregnated microporous membrane has advantageous mechanical properties. Although this approach is successful in improving cell performance and increasing lifetimes, even longer life would be even more desirable.
Various approaches have been used in the art in further attempts to extend membrane life. Shortly after the development of polymer membranes, many practitioners realized that degradation of the membrane occurred through the generation of radical species, for example, peroxy or hydroxy radicals in or near the electrodes. These very active species attacked the polymer and chemically degraded it. Therefore, approaches to reduce or remove these radical species have been developed. For example, it was recognized in the '70s, that “for applications where maximum performance and life are needed, the membrane is treated by depositing a small quantity of catalyst within the solid polymer electrolyte (SPE). The finely divided catalyst, which forms a discontinuous layer, decomposes the small quantity of potentially harmful peroxy degradation species. Also there is a more intimate electrode/electrolyte contact which leads to some performance improvement. The use of the catalyst within the SPE appears to increase membrane life by an order of magnitude compared to untreated material.” [LaConti, et. al., Proceedings of the Symp. On Electrode Materials for Energy Conversion & Storage, McIntyre, J D E; Srinivasan, S; and Will, G G; (eds), The Electrochemical Society, Vol. 77-6, 1977, pg. 354]. Various approaches to achieve such compositions were subsequently developed, for example U.S. Pat. No. 4,959,132 to Fedkiw, U.S. Pat. No. 5,342,494 to Shane, et. al., U.S. Pat. No. 5,472,799 to Watanabe et. al, and U.S. Pat. No. 5,800,938 also to Watanabe.
In '132 a process for producing an in situ metallic electrocatalytic film proximate the surface of a solid polymer electrolyte membrane to form a composite structure useful in promoting electrochemical reactions in fuel cells, sensors, chloralkali processes, dialysis, or electrochemical synthesis cells is described. The method comprises the steps of: loading metal ions into the ionomer matrix of a solid polymer electrolyte membrane to achieve a loading level of metal ions sufficient for forming an electronically coherent film of metal within the ionomer matrix, said metal ions being selected as those which will constitute the chemical composition of the electrocatalytic film; and exposing at least one face of the metal-ion-loaded membrane to a chemical reductant under controlled conditions of time and temperature sufficient to cause the metal ions in the membrane to diffuse towards the exposed face and to be reduced to the metal (0) state while within the membrane, and to produce in situ within the ionomer matrix of the membrane an electronically coherent porous film of metal located predominately within the membrane and near its surface, the electronically coherent film being comprised of metal particles in electrical contact with one another. Although the process described in '132 does describe a process to form an electrocatalytic film proximate the surface of a solid polymer electrolyte membrane, it is a porous film in the membrane, and therefore is less useful in reducing cross-over of hydrogen through the membrane. Furthermore, only unsupported electrocatalyst metal ions are described.
In '494, another method for forming a catalyst impregnated fluorocarbon ion exchange membrane is described. It comprises the steps of: (a) conditioning the ion exchange membrane by exchanging hydrogen ions in the membrane with replacement cations; (b) exchanging said replacement cations with platinum catalyst ions; (c) reducing said catalyst ions to platinum metal; (d) repeating steps “a” through “c” at least once to form a multiply impregnated membrane; and (e) exchanging any remaining replacement cations in said multiply impregnated membrane with hydrogen and (f) equilibrating said multiply impregnated membrane wherein the platinum metal is present in the form of discrete and isolated particles within the membrane. This patent involves multiple complex steps, and produces discrete and isolated platinum metal particles that are not supported.
In '799, a solid polymer electrolyte fuel cell is described. It comprises a cathode current collector, a cathode connected to the cathode current collector, an ion exchange membrane having a catalyst layer; an anode and an anode current collector connected to the anode, the catalyst layer being electronically insulated from the current collectors. This catalyst layer is produced by dipping in an aqueous solution of a platinum amino salt to ion-exchange the exchange groups of the ion exchange resin in the electrodes with the platinum cation, and then the catalyst metal is supported in the vicinity of the surface by reducing the platinum ion by means of such a reducing agent as hydrazine. [col 1, lns. 62-67]. Only unsupported platinum metal catalysts are described, and the catalyst layer is separated from the cathode by an intervening layer of ion exchange membrane [FIG. 2]. In a later issued patents, U.S. Pat. No. 5,766,787 to the same author, a solid polymer composition comprising solid polymer electrolyte selected from cation exchange resin and anion exchange resin, and 0.01 to 80% in weight of at least one metal catalyst selected from the group consisting of platinum, gold, palladium, rhodium, iridium and ruthenium based on the weight of the solid polymer electrolyte contained in the said solid polymer electrolyte is claimed. This patent also only describes unsupported precious metal catalysts in the solid polymer electrolyte, and discloses a similar process as used in '799 to produce them.
In '938, a sandwich-type solid polymer electrolyte fuel cell is claimed. In this patent, a platinum layer (i.e. reaction catalyst layer, 7 in FIG. 2 of '938) was formed by means of sputtering onto a hydrocarbon ion exchange membrane on the anode side having a thickness of 50 microns and an EW value of 900. A commercially available perfluorocarbon-type ion exchange resin solution (“Nafion” solution) was applied on the catalyst layer on the anode side of the ion exchange membrane and dried at 60 degrees C. to form an ion exchange membrane having a catalyst layer whose total thickness was 60 microns [col. 6, lns. 42-52]. Additionally, it is disclosed that a catalyst metal particle (29 in FIG. 4 of '938) can be present in the ion exchange resin (27 or FIG. 4 of '938) of the cathode (24 of FIG. 4 of '938). The latter embodiment (FIG. 4 of '938) only has unsupported metal catalyst particles in the cathode, while the former embodiment (FIG. 2 of '938) discloses only unsupported metal catalyst particles in a layer separated from the cathode by an ion exchange resin (8 in FIG. 2 of '938).
A similar approach is disclosed in U.S. Pat. No. 6,630,263 to McElroy et. al. In this patent, a fuel cell is described, comprising: a cathode flow field plate; an anode flow field plate; an anode catalyst; a cathode catalyst; and a proton exchange membrane. The proton exchange membrane, comprises a catalyst material; and a proton exchange material, wherein the catalyst material is incorporated in the proton exchange material, the cathode catalyst is between the proton exchange membrane and the cathode flow field plate, the proton exchange membrane is between the cathode and anode catalysts, and a planar area of the cathode catalyst is from about 90% to about 99.9% of a planar area of the anode catalyst. In this application, the catalyst material is “a metal or an alloy, such as platinum or platinum containing alloy” [Col 4, line 61-62], and the importance of using a cathode catalyst area smaller than the anode catalyst area is taught. The concept of using a supported catalyst is not disclosed. Although the use of a reinforcement in the proton exchange membrane is disclosed [FIG. 4], the importance of a strong solid polymer electrolyte in combination with the presence of a supported catalyst in the solid polymer electrolyte is not described.
In yet another similar approach U.S. Patent Application 20050175886 to Fukuda, et. al. describes a process for producing an active solid polymer electrolyte membrane comprising: immersing an electrolyte membrane element into a mixture of a noble metal complex solution and at least one additive selected from a water-soluble organic solvent, a nonionic surfactant and a non-metallic base to conduct an ion-exchanging; washing the electrolyte membrane element with pure water; subjecting the electrolyte membrane element to a reducing treatment; washing the electrolyte membrane element with pure water; and drying the electrolyte membrane element; wherein the active solid polymer electrolyte membrane comprises a solid polymer electrolyte element, and a plurality of noble metal catalyst grains which are carried by an ion exchange in a surface layer located inside a surface of said solid polymer electrolyte element and which are dispersed uniformly in the entire surface layer, said surface layer having a thickness t2 equal to or smaller than 10 microns, wherein an amount CA of noble metal catalyst grains carried is in a range of 0.02 mg/cm2.≦CA≦0.14 mg/cm2. The method described in '886 the surface layer as “noble metal catalyst grains” [col 2, ln. 1]. Further, the method embodied in the claims is not capable of producing supported catalysts, which are present within the current invention.
In addition to the approaches described above, others have described alternative approaches to modifying the membrane in solid polymer electrolyte fuel cells. These include U.S. Pat. No. 6,335,442 to Asukabe, et. al., JP 2001-118591 to Morimoto, et. al., US Patent Application 2003/0008196 to Wessel, et. al., and European Patent Application EP 1289041 A2 to Iwassaki et. al, In these applications, solid polymer electrolytes comprising various alternatives to precious metal catalysts are claimed. For example, in '442 a solid polymer electrolyte membrane comprising oxide catalysts and macrocyclic metal complex catalysts is claimed. Similarly, in JP2001-118591 transition-metal oxides are disclosed as useful catalysts in solid-state polyelectrolytes; in '0008196 salts, oxides or organometallic complexes of group 4 elements are claimed; while in EP 1289041 antioxidants containing tri-valent phosphorus or sulfer are suggested. In none of these cases is the formation of a layer in the solid polymer electrolyte of supported precious metal catalyst disclosed, nor is the importance of the mechanical properties of the membrane.
More recently, additional art in US Patent Application 2004/0043283 to Cippollini, et al.; US Patent Application No. 2004/0095355 to Leistra, et. al.; and US Patent Applications 2004/0224216 and 2005/0196661 to Burlatsky et. al. has published. In '43283, a membrane electrode assembly, comprising: an anode including a hydrogen oxidation catalyst; a cathode; a membrane disposed between said anode and said cathode; and a peroxide decomposition catalyst positioned in at least one position selected from the group consisting of said anode, said cathode, a layer between said anode and said membrane and a layer between said cathode and said membrane, wherein said peroxide decomposition catalyst has selectivity when exposed to hydrogen peroxide toward reactions which form benign products from said hydrogen peroxide. In '95355 a method for making membrane electrode assemblies such as those described in '43283 is claimed. In '224216, a membrane electrode assembly, comprising: an anode; a cathode; a membrane disposed between the anode and the cathode; and an extended catalyzed layer between the membrane and at least one electrode of the anode and the cathode, the extended catalyzed layer comprising catalyst particles embedded in membrane material and including a plurality of particles which are electrically connected to the at least one electrode. Similarly, in '196661, a membrane electrode assembly, comprising: an anode; a cathode; a membrane disposed between the anode and the cathode; and an extended catalyzed layer between the cathode and the membrane, the extended catalyzed layer being adapted to reduce oxygen, and decompose hydrogen peroxide and free radicals to produce water.
In all four of these applications, a peroxide decomposition catalyst is present, and that catalyst either “has selectivity when exposed to hydrogen peroxide” ('95355 e.g., claims 1, 25, and Paragraphs 8; and '43283, e.g., claims 1, 10, 26, 33 and Paragraphs 8, 9 &10) or is “electrically connected to cathode” '(196661, Paragraph 23) or “to at least one electrode” ('224216, Paragraph 10). In all four cases, the layer is shown as part of an extended electrode (e.g., FIG. 1a in '224216, FIG. 4 in '95355 and '43283 and FIG. 3 in '196661). In '95355 and '43283 the peroxide decomposition catalyst may also be disposed in a separate layer (70 in FIG. 6 in '95355 and '43283) by being dispersed through the layer. In this case though, the membrane is homogeneous outside of dispersed peroxide decomposition catalyst layer, (as shown in FIG. 6 in '95355 and'43283). Further, the critically important role of the mechanical properties of the membrane discovered herein is not disclosed, nor are any specific characteristics of the dispersed peroxide decomposition catalyst disclosed.
Additional related art has focused on hydrating the membrane through the use of various solid particles is given by U.S. Pat. No. 5,203,978 to Tsou, et al.; and to Mathias et. al., in U.S. Pat. No. 6,824,909. In each of these an inorganic particle such as a boride, carbide or nitride of a Groups IIIB, IVA, IVB, VB, and VIB metal ('978) or a zeolite ('909) is present. In '978 no catalyst is present, while in '909 a catalyst is present, but only on “adsorbent particles embedded in the membrane which adsorb water under wet conditions” [col. 2, ln. 8-10]. Non-absorbing particles, for example carbon, are not considered described.
In JP 2003-123777 to Takabe, et. al., a polymer electrolyte fuel cell comprising a hydrogen ion conductive polymer electrolyte membrane, and a pair of separators having gas flow channels whereby fuel gas is supplied to and discharged from one of the electrodes, and antioxidant gas is supplied to and discharged from the other, wherein said fuel cell is characterized in that the electrodes are provided with catalyst layers in contact with the hydrogen ion conductive polymer electrolyte membrane, and at least one of the catalyst layers of the electrodes has a hydrogen ion conductive polymer electrolyte, electroconductive carbon particles that support the catalyst particles, and a peroxide decomposition catalyst is claimed. In the specification and working examples of '123777, the inventors emphasize the importance of electrical isolation of the peroxide decomposition catalyst from the electrode. For example, “it is also effective to electronically insulate the space between the peroxide decomposition catalyst and the electrodes . . . ” (Paragraph 14), and “it is also effective for the peroxide decomposition catalyst to be supported on electrically insulating particles. (also Paragraph 14]”. In fact, the inventors in '123777 go to great lengths to provide an electrically isolated peroxide decomposition catalyst, for example by mixing Pt/carbon catalyst with an ionomer solution followed by drying, hardening, and crushing of the mixture (Working Example 1). We have discovered, surprisingly, that such electrical isolation is not required to extend life and reduce degradation of solid polymer electrolytes. In this application, as described more fully below, a substantially occlusive, electronically insulating composite layer is present, but the individual catalyst on supporting particles do not need to be electrically isolated from the electrode as taught by Takabe, et. al. In fact, carbon support particles, which are electrically conductive, are effective in the instant invention of this application without the additional treatments required by Takabe et. al. as described in his Working Example 1.
During normal operation of a fuel cell or stack the power density typically decreases as the operation time goes up. This decrease, described by various practitioners as voltage decay, fuel cell durability, or fuel cell stability, is not desirable because less useful work is obtained as the cell ages during use. Ultimately, the cell or stack will eventually produce so little power that it is no longer useful at all. Furthermore, during operation the amount of fuel (e.g., hydrogen) that crosses over from the fuel side to the oxidizing side of the cell will increase as the health of the membrane deteriorates. In this application, this hydrogen cross-over will be used to determine membrane life.
A life test is generally performed under a given set of operating conditions for a fixed period of time. The test is performed under a known temperature, relative humidity, flow rate and pressure of inlet gases, and is done either in fixing the current or the voltage. In this application, the life tests are performed under constant current conditions, though it is well known in the art that constant voltage life tests will also produce decay in the power output of a cell. Herein, life is determined by temporarily stopping a life test, i.e., removing the cell from external load, and then determining the level of hydrogen cross-over in the cell. If the hydrogen cross-over is above some predetermined level, 2.5 cm3 H2/min is used herein, then the test is ended, and the life is calculated as the number of hours the cell has operated. (Specific details of the test protocol used herein for life determination are described below).
Although there have been many improvements to fuel cells in an effort to improve life of fuel cells, there continues to be an unmet need for even more durable fuel cells, and in particular, more durable membrane materials for use in PEMFCs.